SQL Server 2017

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create table Table1 (firstdate date, lastdate date );
SELECT T.firstdate
       , T.lastdate 
       , (T.lastdate - T.firstdate) as ALIAS1
       , case when (T.lastdate - T.firstdate) > 15 then 'a' else 'b' end as ALIAS2 
FROM Table1 T;
Msg 8117 Level 16 State 1 Line 3
Operand data type date is invalid for subtract operator.

select t.firstdate, t.lastdate, tt.alias1,
       (case when alias1 > 15 then 'a' else 'b' end) as alias2
from table1 t cross apply
     ( values (t.lastdate - t.firstdate) 
     ) tt(alias1);
Msg 8117 Level 16 State 1 Line 4
Operand data type date is invalid for subtract operator.

SELECT T.firstdate
       , T.lastdate 
       , T.lastdate - T.firstdate as ALIAS1
FROM Table1 T;
Msg 8117 Level 16 State 1 Line 3
Operand data type date is invalid for subtract operator.

SELECT T.firstdate
       , T.lastdate 
       , T.lastdate - T.firstdate as ALIAS1
FROM Table1;
Msg 4104 Level 16 State 1 Line 1
The multi-part identifier "T.firstdate" could not be bound.
Msg 4104 Level 16 State 1 Line 2
The multi-part identifier "T.lastdate" could not be bound.
Msg 4104 Level 16 State 1 Line 3
The multi-part identifier "T.lastdate" could not be bound.
Msg 4104 Level 16 State 1 Line 3
The multi-part identifier "T.firstdate" could not be bound.

SELECT T.firstdate
       , T.lastdate 
       , 3 - 1 as ALIAS1,
case when ALIAS1> 15 then 'a' else 'b' end as ALIAS2 
FROM Table1 T;
Msg 207 Level 16 State 1 Line 4
Invalid column name 'ALIAS1'.